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Quick and Dirty DDL Laser Driver

Submitted by Braden on Fri, 01/13/2012 - 12:51am

When using a laser diode it isn't advisable to hook up a power source and run your delicate diode directly (see what I did there?).Though it is possible to drive the laser diode using a pair of AA batteries and a resistor, you are asking for trouble down the line; and the same goes for an unregulated power supply, even if it is the right voltage. With a diode you must both regulate the voltage and restrict the current. Diodes are notoriously current hungry, unlike most electronic devices. Typically a component will pull the amount of current it needs and stop there. On the contrary, often times a diode will just pull more and more curerent until it burns itself out. This is why you need to use a laser driver with any laser diode. This leaves us with two options: go to ebay and order one that is neatly soldered to a breadboard and sealed in a package (while paying significantly more, waiting 'x' amount of time for shipping, and not having fun with a 700 degree torture device), or make a quick trip to Radioshack and build one with ease. I obviously chose the second option, hence this article. Lets begin.

The capacitor acts as a filter to smooth ripple. This is especially important in the case of an unregulated power supply being used, such as a "wall wart".

The diode provides protection to the laser in the event that the power supply is hooked up backwards. If it doesn't turn on, this is likely why.

To calculate the fixed resistor values:

Find the datasheet for your laser diode and locate the following:

Operating Current (Max current) - This is 'I'

Use the equation V = I*R (Ohms law)

We need to find R so... R = V/I

The LM317 reference voltage is always 1.25v - This is 'V'

Plug the values into the equation and calculate.
- The number you get will be in kOhms, we need ohms

Multiply the result you from above by 1000. This is the resistor value you will need.

If you don't have the resistor size needed, the resistors can be wired in parallel to decrease the value or in series to increase the values. The equations used to calculate these are:

Series:RTOTAL = R1 + R2 + R3 + ...

Parallel:1 / RTOTAL = (1 / R1) + (1 / R2) + (1 / R3) + ...

To calculate the necessary input voltage:
Add the LED operating voltage from the data sheet to 3v (for the regulator) and this is the power required to operate the laser. If running on battery add another 1.2v, this will give a much better battery longevity.

Now that you have all of your parts, simply follow the schematic below to build the constant current (DDL) laser driver.

Before hooking up your laser... If you applied power to this circuit at any time, remember to short the leads of the capacitor. If you fail to do this it will kill the laser when the stored voltage is discharged. This is only necessary if you applied power without a load to consume the energy when it is turned off.

In addition. You will more than likely want to add some sort of power switch to this device. It MUST be placed between the power supply and the driver, NOT the driver and the laser diode. This will have the same effect as not discharging the capacitor when it is turned on and will damage the laser diode.

If everything goes well you will now have your own working laser driver and a happy diode! To finish it off, if you haven't already, pull the pieces from the breadboard and solder them NEATLY to a proto board. Radioshack sells general purpose ones that are more than useable, but you can design and print your own if you so choose.

The fine print
* Remember, you are working with lasers here, they are harmful if aimed in the eyes; some may even be able to cause severe burns or start fires. Do not do anything that would make your mother say "Name, what in heavens sakes were you thinking? Why would you ever do 'insert world ending action here'?" In other words, be responsible, obey any stickers/warnings, and don't break any laws. *

** Be sure to check your voltages and current with a multimeter before connecting a laser diode. I am not responsible for any damages or fatalities caused by this article. I used these exact steps to make several of my own successfully, but yet, it comes with no warranty. **